Die-furnace, especially for the fabrication of sintered products

ABSTRACT

The die-furnace comprises a cylindrical die of refractory material, a banding assembly which surrounds the die externally and transmits a radial compressive force to the die by means of packing material, an induction heating device located within the packing material and constituted by a cooled primary coil connected to an external high-frequency power supply and by a coaxial secondary circuit which surrounds the die and in which a current is induced by the primary coil.

[ 1 July 25, 1972 United States Patent Carcey 77 man 33 11. 0 1 a/ 9 1"1 2,908,739 10/1959 Rummel 3,311,695 3/1967 Kasper 3,303,258 2/1967Junkev...........................

FOREIGN PATENTS OR APPLICATIONS [54] DIE-FURNACE, ESPECIALLY FOR THEFABRICATION OF SINTERED PRODUCTS [72] Inventor:

France Jacques Carcey, Rives, France [73] Assignee: Commissariat ALEmergie Atomique,

Paris, France May 10, 1971 eynolds norney-Cameron, Kerkam & SuttonABSTRACT Primary Examiner-J. V. Truhe Assistant Examiner-B. A. R

[22] Filed:

[21] Appl. No.:

ounds the die and in which a current isi ry coil.

nduced by the prima- References Cited 5 Claims, 2 Drawing Figures UNITEDSTATES PATENTS 2,437,127 3/1948Richardson............................219/149 DIE-FURNACE, ESPECIALLYFOR THE FABRICATION OF SINTERED PRODUCTS This invention relates to anapparatus comprising a pressure die which is primarily intended topermit the compression of a powder to be sintered and a furnace which iscombined with the die and serves to heat said powder to a hightemperature, all the conditions which are necessary for the sinteringprocess being consequently brought together within a Single apparatus.

The chief aim of the invention is to increase the performances ofconventional dies, to permit at the same time the possibility of alarger geometry for the parts employed and, while improving theuniformity of temperatures through the sintered product, to limit thethermal gradients between the heating source and the product through thedie itself, thereby reducing the differences in thermal expansion duringoperation as a result of temperature variations. A further object of theinvention is to provide a simple and relatively inexpensive assemblycomprising a die of refractory material which is capable of beingemployed a large number of times without any danger of seizure, inparticular at the time of displacement of plungers which carry outwithin the die the compression of the product to be sintered. Finally, afurther object of the invention is to permit immediate application ofthe die-furnace under consideration to all the usual sintering processesand especially to those in which the compression of the product to besintered is an isostatic compression obtained by means of a fusiblemetal or pressure-transmitting agent as well as to the processes inwhich said isostatic compression is accompanied by mechanical creep orplastic flow as a result of deformation and reduction in volume of anintermediate casing.

To this end, the die-furnace under consideration essentially comprises adie of a type known per se and constituted by a cylindrical element ofrefractory material, a banding assembly which surrounds the dieexternally, packing material placed between the die and the bandingassembly for transmitting to the die the radial force which is producedby said assembly and an induction heating device located within thepacking material and constituted by a primary winding having circularturns connected externally of the die-furnace to a highfrequencyelectric power supply and a completely closed secondary circuit in whicha current is induced by the primary winding and which surrounds the diewhile being co-axial with said primary winding.

Apart from this main characteristic feature, a die-furnace asconstructed in accordance with the invention also has a number of otherrelated features which are preferably to be considered in combinationbut could also be considered separately and are primarily concerned withthe following aspects:

the banding assembly is constituted by a series of superposed open loopsinsulated from each other and associated with mechanical means forpositional locking of each loop with respect to the next in order toform a rigid non-deformable block the primary winding of the heatingdevice is constituted by a tubular inductor which is cooled inparticular by water, the primary circuit being passed through thebanding assembly by means of a coaxial passage, the external portion ofsaid passage being connected to ground in order to prevent inductionphenomena within the banding assembly the secondary circuit comprises atleast two concentric loops in closed relation the metallic sleeve of thesecondary circuit is provided with cut-out portions in order that thecompression forces applied by the banding assembly should be transmittedto the die by means of the packing material the packing material isconstituted by a refractory substance and especially by concrete madefrom an insulating cement mixed with grains of high-density alumina.Advantageously, the concrete which is employed has a variableparticle-size distribution within its mass between the banding assemblyand the die the die-furnace is rigidly fixed to the cooled internal wallof a leak-tight outer enclosure for creating a vacuum, said enclosurebeing provided with an opening for establishing a communication with apump set and dynamic seals providing a leak-tight passage through theenclosure wall for plungers which serve to compress the product to besintered and penetrate axially into the die the product to be sinteredcan be surrounded if necessary by a deformable casing which is placeddirectly within the die.

Further properties of a die-furnace in accordance with the inventionwill be brought out by the following description of one exemplifiedembodiment and of a number of modes of application, reference being madeto the accompanying drawings in which FIG. 1 is a view in perspectiveand in partial cross-section showing the die-furnace under considerationFIG. 2 is an axial half-sectional view of said die-furnace.

In FIG. 1, the reference numeral 1 designates the apparatus ordie-furnace as a whole comprising a die 2 of a type which is known perse and designed in the form of a heat-resisting circular cylinder whichis open at both ends and formed of sintered alumina or any othermaterial which is capable of carrying out this function. A product 3 tobe sintered at a suitable temperature and pressure is placed within saidcylinder and is either a ceramic or metallic refractory powder which hasbeen pre-densified in the cold state in the most suitable manner or apowder of the same type which is placed within a sealed staticvacuumenclosure. By reason of the high axial pressures to which the product tobe sintered is subjected within the die 2 and which give rise inparticular to deformation of the product so that an appreciable radialforce is applied by said product against the internal surface of thedie, said die must be maintained externally in order that it may bepermitted to withstand the stresses developed therein.

To this end and in accordance with the invention, the diefumace underconsideration is provided at its periphery with a banding assembly 4which will be described in greater constructional detail with referenceto FIG. 2, said assembly being so arranged that a system of forcesdirected towards the axis of the die 2 is continuously applied to thislatter. The region which is located between the die 2 and the bandingassembly 4 is filled with packing material 5 preferably consisting ofrefractory concrete which, in the exemplified embodiment hereindescribed, is formed of insulating cement and grains of high-densityalumina.

Moreover, in accordance with another essential arrangement of theinvention, the die-furnace l is associated with an induction heatingdevice constituted by a primary winding 6 and a completely closedsecondary circuit which is formed in the embodiment illustrated in FIG.1 by means of two coaxial sleeves 7 and 8 which surround the die 2 andare joined together by means of radial portions 9 and 10. The primarywinding 6 is preferably of the cooled inductor type consisting of atubular conductor 12, a cooling fluid 11 which is preferably water beingcirculated within said conductor. The primary winding 6 is passedthrough the banding assembly 4 by means of a coaxial device 14 which isalso cooled the highfrequency current is admitted at the center thereturn to the exterior which is insulated from the admission by means ofa suitable insulator 13 is preferably the high-frequency ground. Thispenetration device prevents any stray induction within the bandingassembly 4.

Reference will now be made to FIG. 2 which shows more precisely a fewconstructional details of the die-furnace under consideration. inparticular, it is apparent from this figure that the external bandingassembly 4 is constituted by means of a series of open and superposedloops 15 which are metallic, for example, and insulated from each other,said loops being disposed in an irregular manner and associated withinterlocking means constituted by the packing material and/or parts ofhigh-density alumina which serve to secure the abovementioned loops withrespect to each other in order to constitute a rigid non-deformableblock at the periphery of the die-furnace. In order that the bandingforces arising from the above-mentioned assembly 4 should be transmittedthrough the packing material 5 to the die 2, the sleeves 7 and 8 whichconstitute the secondary circuit of the heating device are provided witha series of openings such as 17 and 18 which are suitably distributedacross the surface of said sleeves.

The die-furnace can also be arranged so that the external surface of thebanding assembly 4 should be coupled to the wall 19 of a sealedenclosure 20 which completely surrounds said assembly and is connectedby means of a duct 21 to a pump set (not illustrated) which serves tocreate a vacuum within the enclosure and especially around the product 3to be sintered which is placed within the die 2. In order to carry outthe compression of the product 3, the enclosure 20 is associated withtwo plungers which are designated respectively by the reference numerals22 and 23 and pass through the end-walls 24 and 25 of said enclosurewith interposition of dynamic seals 26 and 27, said plungers 22 and 23being adapted to engage axially within the die 2. Finally,temperature-measuring instruments and especially thermocouples 28 areplaced at different points of the die-furnace l and especially withinblind-end bores 29 which are formed within the die 2 itself.

In the example described in the foregoing in which the diefurnace l isplaced within the interior of a sealed enclosure 20 for compressing theproduct 3 in a dynamic vacuum by virtue of the seals 26 and 27 thepractical construction of the equipment makes it necessary to ensurethat the winding 6 should pass through the enclosure wall by means of avacuum-tight coaxial passage 30. By way of alternative and in anessentially equivalent manner insofar as the basic design of thedie-furnace is concerned, the sealed enclosure 20 could be dispensedwith and the product 3 to be sintered could be simply placed within aclosed vacuum-type casing 31 which maintains a static vacuum within theproduct, in which case the die-furnace is placed in the surroundingatmosphere. In the alternative embodiment just mentioned, thecompression of the product 3 is naturally carried out in a staticvacuum.

The die-furnace as hereinabove described makes it possible to carry outwith suitable values of pressure and temperature any known sinteringtechnique and in particular the techniques which are contemplated inFrench Pat. No. 1 548 603 as filed in the name of Commissariat alEnergie Atomique.

In the first case, the axial thrust of the plungers which penetrate intothe die is converted by means of a metal which is capable of melting atthe operating temperature into an isostatic pressure on the productwhich is accordingly subjected to simple compression, thereby resultingin rearrangement of the product by plastic flow. In the second case, theaxial thrust is converted by means of a pressure-transmitting agentwhich can be either a fusible metal or a refractory powder at theoperating temperature into isostatic compression on the product togetherwith a plastic flow action produced by means of a deformation andreduction in volume of a deformable casing which surrounds the productto be sintered. As has already been indicated, the above mentionedoperations can be carried out according to requirements either in adynamic vacuum or in a static vacuum the last-mentioned solution has theadvantage of simplification of the apparatus as a whole and especiallyof removing the problems involved in degassing of the packing materialof the die-furnace itself. In the case of the method which resultssimultaneously in isostatic compression and in plastic flow, the productto be sintered under pressure is advantageously pre-densified in thecold state and in a vacuum, then placed within a compression casingwhich is deformable at operating temperatures, said casing beingsurrounded by suitable envelopes or shells which contain apressure-transmitting agent and prevent any diffusion of this latterinto the product itself this second method can be carried out either ina static or dynamic vacuum.

Whatever method may be chosen, the action of the plungers 22 and 23 onthe product 3 within the die 2 produces high radial thrust forces on theinternal surface of said die. Owing to the nature of the refractorymaterial which constitutes the die, this latter usually has low tensilestrength and consequently high mechanical fragility. This disadvantageis completely removed by virtue of the external banding assembly 4 andby reason of the fact that this latter produces action on the die 2through the intermediary of the packing material 5 while applying radialforces which are readily transmitted through the openings 17 and 18formed in the sleeves 7 and 8 of the secondary heating circuit.

The die-furnace under consideration has another significant advantagewhich arises from the particular design concept of the heating deviceitself and makes it possible especially to limit the value of thethermal gradient within the die 2 proper. In point of fact, the heatbuild-up in the charging loop of the secondary circuit as a result of aresistive effect and the simultaneous heat build-up in the assemblyconsisting of product and casing under the induction heating effectmakes it possible to maintain the die in a uniform temperature zone.Furthermore, the product itself may be heated by induction by means ofthe secondary circuit on condition that said product has the property ofconductivity at the operating frequency, in which case the heat builduptakes place within the product itself.

The die-furnace under consideration makes it possible under theconditions described in the foregoing to contemplate either continuousor non-continuous sintering of any ceramic, refractory or metallicmaterial at high temperatures which can attain 1500" C or more underpressures ranging from a few Kg/cm" to 2 T/cm or more. Moreover, thedesign concept of the apparatus as a whole is such that hot delivery ofthe sintered product from the mold can readily be carried out whileretaining said product in a zone which is not subjected to mechanicalstresses during cooling. The furnace makes it possible to form on theproduct a uniform-temperature zone which may be localized within theproduct itself if necessary. The structural design of the primarywinding and of its grounded coaxial passage within the banding assemblyprevents stray-induction effects, in particular within the loops of thebanding assembly. The packing material can be chosen so as to have adifferent particle-size distribution according to the zones which areoccupied and to constitute in particular an insulating material whichplays a contributory role in improving the interlocking of the loops ofsaid assembly.

A number of other advantages also arise from the foregoing descriptionof the die-furnace and the most significant of these are as follows vThe design concept of the heating device permits efficient banding byvirtue of its transformer and cut-out portions while at the same timecarrying out either direct sintering of metallic materials underpressure or alternatively sintering of refractory or ceramic substancesif use is made of a sealed vacuum enclosure or an unsealed enclosure ofmetallic type. The die-furnace also has greater potentialities on theone hand as a result of banding by consolidation through the heatingdevice and on the other hand by virtue of the uniformity of temperaturesin the die and in the product as achieved by simultaneous heat build-upin the charging loop of the secondary circuit and in the assemblyconsisting of product and casing. Hot take-out, continuous pressing,incorporation with a pressing and evacuating system or incorporationwith a standardized modular assembly for hot-pressing can becontemplated.

It will clearly be understood that the invention is not limited in anysense to the exemplified embodiment which has been more especiallydescribed with reference to the accompanying drawings but extends on thecontrary to all alternative forms.

What we claim is: 1

1. A die-furnace especially for the fabrication of sintered products,comprising a die of a type known per se of a cylindrical element ofrefractory material, a banding assembly surrounding said die externally,packing material between said die and said banding assembly transmittingto said die the radial force produced by said assembly, an inductionheating device within said packing material a primary winding for saiddevice having circular turns connected externally of the die-furnace toa high-frequency electric power supply and a completely closed secondarycircuit for said device in which a current is induced by said primarywinding and surrounding and heating the external surface of said die andcoaxial with said primary winding.

2. A die-furnace according to claim 1, wherein said banding assemblycomprises a plurality of superposed open loops insulated from each otherand interlocked by said packing material to form a rigid non-deformableblock.

3. A die-furnace according to claim 1, wherein said secondary circuitincludes at least two concentric loops in closed relation.

4. A die-furnace according to claim 3, wherein said loops are a metallicsleeve having cut-out portions whereby the compressive forces producedby said banding assembly are transmitted to said die by said packingmaterial.

5. A die-furnace according to claim 1, said die-furnace being rigidlyfixed to the cooled internal wall of a leak-tight outer enclosuresupporting a vacuum, an opening in said enclosure communicating with apump set and a leak-tight passage through said enclosure for plungerscompressing the product to be sintered and axially entering said die.

1. A die-furnace especially for the fabrication of sintered products,comprising a die of a type known per se of a cylindrical element ofrefractory material, a banding assembly surrounding said die externally,packing material between said die and said banding assembly transmittingto said die the radial force produced by said assembly, an inductionheating device within said packing material , a primary winding for saiddevice having circular turns connected externally of the die-furnace toa high-frequency electric power supply and a completely closed secondarycircuit for said device in which a current is induced by said primarywinding and surrounding and heating the external surface of said die andcoaxial with said primary winding.
 2. A die-furnace according to claim1, wherein said banding assembly comprises a plurality of superposedopen loops insulated from each other and interlocked by said packingmaterial to form a rigid non-deformable block.
 3. A die-furnaceaccording to claim 1, wherein said secondary circuit includes at leasttwo concentric loops in closed relation.
 4. A die-furnace according toclaim 3, wherein said loops are a metallic sleeve having cut-outportions whereby the compressive forces produced by said bandingassembly are transmitted to said die by said packing material.
 5. Adie-furnace according to claim 1, said die-furnace being rigidly fixedto the cooled internal wall of a leak-tight outer enclosure supporting avacuum, an opening in said enclosure communicating with a pump set and aleak-tight passage through said enclosure for plungers compressing theproduct to be sintered and axially entering said die.